Polycarbonates are well-known thermoplastic materials finding a wide range of uses, particularly for injection molding applications. However, many of the ingredients which go into the preparation of the polycarbonate, contain metallic contaminants such as nickel, copper and iron. These contaminants cause the polycarbonate to be thermally unstable by affecting the initial color and the color stability of the polycarbonate under molding and or heating conditions. The polycarbonate is so affected when the amount of the contaminant exceeds about 0.2 parts per million. Since polycarbonates are used in the manufacture of molded products, this discoloration and instability which occurs upon, in, or after molding is very objectionable. Several attempts have been made to correct these problems; however, most of these attempts have somewhat limited success. Heating or molding of polycarbonate polymers containing these metallic contaminants has invariably resulted in discoloration of the product.
It has been discovered that by adding gluconic acid or its alkali metal salts during the preparation of the polycarbonate, color and thermally stable polycarbonates are produced.